Below, the background art of the present invention will be explained.
The post-coated metal sheet which is coated after being shaped for use in home electrical appliances, building materials, automobiles, etc. is being replaced by precoated metal sheet which is covered with a colored organic film. A precoated metal sheet, in most cases, has excellent corrosion resistance and workability and coating film adhesion due to the composite effects of the metal itself (in the case of a plated metal sheet, the plating film) and the chemical conversion at the layer above it and, furthermore, the primer film at the layer above that and, furthermore, in many cases is provided with a colored organic film at its top surface, so the coating operation after working can be eliminated and a high productivity and beautiful appearance can be obtained.
When press-forming a precoated metal sheet, the film layer which is covered over the metal sheet is also shaped, so the film is also required to have workability. For this reason, a film layer based on a resin is the general practice. The covering film of a precoated metal sheet is usually insulating. However, in precoated metal sheets, a need for electroweldability at the time of assembly of parts and a need for groundability, electromagnetic shieldability, and other high conductivity when used for home electrical appliances or the housings for OA equipment have arisen. In response to these demands for imparting conductivity to films, the art has been proposed of covering a metal sheet with a film which contains conductive particles to thereby impart conductivity.
Among these, as art which uses conductive metal particles, for example, PLT 1 proposes the art of covering an aluminum or aluminum alloy substrate surface with a resin film which contains aluminum or aluminum alloy powder through a chromate film which strengthens the corrosion resistance of the substrate or the adhesion with the substrate so as to thereby obtain a precoated aluminum or aluminum alloy material for home electric appliance products or chassis parts which realizes both excellent corrosion resistance and conductivity. The amount of the aluminum or aluminum alloy powder which is used for the resin film is described as being from 10 to 50 parts by weight with respect to 100 parts by weight of the resin.
PLT 2 proposes the art of a galvannealed steel sheet which has a resin-based conductive coating film which contains zinc powder. It is stated that inclusion of zinc powder in the coating film in 30 to 90 mass % is preferable and that a coating film thickness of 2 to 30 μm is preferable.
PLT 3 proposes the art of using a resin film which contains 2 to 50 mass % of a metal powder, 1 to 50 mass % of water, and 0.5 to 30 mass % of a surfactant as essential ingredients and has a thickness of 5 μm or less so as to cover a metal sheet to raise its conductivity. It is stated that as the metal powder, nickel powder is suitable and as the coating for coating use, a water-based one is preferable.
Further, among the art for imparting conductivity to a film, as art which enables use of conductive particles other than metal particles, for example, PLT 4 discloses the art of an organic composite plated steel sheet which has, on a rust-prevention layer mainly comprised of a chrome compound, an organic resin coating of 0.5 to 20 μm thickness containing 3 to 59 vol % of a conductive powder. PLT 5 proposes the art of an organic coated plated steel sheet which has a resin-based film which contains 3 to 59 vol % of a conductive material. As the conductive material, various metals and their alloys, iron phosphate or ferrosilicon or other iron compounds, etc. may be mentioned. PLT 6 discloses the art of a conductive precoated metal sheet which has a 0.5 to 3 μm thick coating film which contains any conductive metal oxide. As the conductive metal oxide, it is stated that one which includes zinc oxide of a particle size of 5.0 μm or less and an average 2 μm in 40 to 50 parts by mass with respect to 100 parts by mass of resin is desirable. PLT 7 proposes as a metal surface coating agent which enables the formation of a conductive, weldable corrosion resistant film after curing at the metal surface, a water-based coating agent which contains a specific organic binder in 10 to 30 mass % and a conductive material powder in 30 to 60 mass %. As examples of conductive material powder which is suitable for preparation of the present coating agent, zinc, aluminum, graphite, carbon black, molybdenum sulfide, and iron phosphate may be mentioned. PLT 8 proposes the art of automobile-use organic precoated steel sheet which achieves both excellent corrosion resistance and weldability by covering the surface of zinc-based plate steel sheet or aluminum-based plated steel sheet with a first layer film which strengthens the adhesion with the plating and, through that, a resin-based second layer film which includes a rust-proofing additive and a conductive pigment. The conductive pigment is contained in the film in an amount of 5 to 70 vol %. The film thickness is 1 to 30 μm. As a suitable conductive pigment, a metal, alloy, conductive carbon, iron phosphate, carbide, and semiconductor oxide may be illustrated.
Further, as art which uses conductive ceramic particles among conductive particles other than metal particles, for example, PLT 9 proposes the art of a conductive material-coated corrosion resistant metal material which is excellent in corrosion resistance and conductivity comprised of core metal which is covered by a clad layer comprised of a corrosion resistant metal and, furthermore, is covered over that by a surface treatment layer comprised of at least one conductive material selected from a carbon material, conductive ceramic, and metal powder and of any resin which binds the same.